Download Trends Towards Progress of Brains and Sense Organs

Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
Trends Towards Progress of Brains and Sense Organs*
BERNHARD RENSCH
Zoologisches Institut, Miinster (Westf.), West Germany
to diseases; 3) higher absolute (not relative) speed
in attacking or fleeing; 4) more favorable proportions of some structures growing with positive
allometry, for example, relatively larger weapons
(mandibles, chelae, antlers, tusks, etc.); 5) an
absolutely larger number of cells of the relatively
smaller sense organs and brains; 6) in mammals
a relatively, and absolutely larger size of the
forebrain and isocortex, that is to say, of the most
complicated part of the cortex (see below); 7)
more dendritic ramifications of the brain neurons;
8) correspondingly a better learning capability;
9) in warm-blooded animals a smaller loss of
calories by heat radiation, as larger animals have a
relatively smaller surface area than smaller ones;
10) longer duration of life; 11) in poikilothermous
animals a larger number of offspring (greater
number of eggs); 12) in viviparous animals relatively less burdening by young animals in the
uterus because they are relatively smaller (B.
Rensch 1943, 1947, 1954, and especially 1959).
(In trans-specific competition, the selective value
may be much higher with regard to larger differences of body size between competing species).
But to some extent Cope's rule may also depend
upon cumulative effects of heterosis. Normally a
new race or a new species begins in a relatively
small area and this means a relatively small
number of individuals. Corresponding to the
increasing size of the area and the increasing
number of individuals, the quantity of new
mutations and also the multiplicity of recombinations increases. Thus, the possibility of heterosis
effects, especially the increase of body size, also
increases. Such a heterosis may perhaps be the
main factor, which causes the increase of body
size in the lines of descent of Foraminifera where
Cope's rule is valid (compare H. Hiltermann and
W. Koch 1950) but where all the selective factors
mentioned before seem not to be effective.
If now the inherited body size is increased by
one of the mentioned factors or by a combination
of some of them, the proportions of most organs
and structures will be shifted as they grow with
positive or negative allometry with regard to
body size. In such cases natural selection may
also produce disadvanatgeous characters if the
disadvantage is much smaller than the advantage
of the increased body size. For example, it may
be disadvantageous when in large animals the
bones become too bulky and tusks or antlers too
large. However, during the ice age it was probably
of greater importance to have a relatively large
body which loses relatively less heat because of
CHARACTERISTICS AND TYPES OF TRENDS
Many mutations primarily produce quantitative differences. They may intensify or slow
down, prolong or abbreviate the development of a
special structure, of an organ, or of the whole
individual. Such quantitative alterations of developmental processes are finally caused by the
uni-dimensionality of the course of time. When the
direction of selection remains constant during a
longer period of phylogeny, as is often the case,
then special trends of evolution will result. Hence,
we often see an increase or a decrease of the
relative size of a structure or an organ or of the
body size as a whole. By the addition of such purely
quantitative alterations, a structure may finally
reach another level of integration distinguished
qualitatively by new characters or functions. In
a parallel manner, the integration of pure quantitative alterations of the number of atoms leads to
new molecules having new qualities.
The parts of a body are connected by numerous
correlations. Therefore, all quantitative alterations show many secondary effects in other
structures or functions, and all trends, which are
caused by quantitative mutations and by a
constant selection, will cause "secondary trends",
as D. M. S. Watson (1949) called them. Especially
the increase or decrease of body size will shift
many proportions of organs or structures and,
hence, also many functions. As in many cases the
process of speciation is accompanied by a change
in body size, the analysis of the correlations
involved will be of special interest.
In many groups of non-flying animals, especially
in most lines of descent among mammals, we may
observe a general trend towards successive
increase of body size, known as Cope's rule or
Cope-D@~r6t's rule. Of course we are not here
concerned with "rectilinear" alterations or with
a real "orthogenesis", as G. G. Simpson emphasized (1949, 1953). It is ony a general tendency
which remains constant, whereas the intensity
of the alterations may increase or decrease in
some periods according to special adaptations to
changes of environment.
Apparently this rule is caused by the higher
selective value of larger varieties when they
compete with smaller ones or of larger species
when they compete with smaller species. Larger
animals normally show: 1) greater physical
strength which is important among competing
embryos and young animals; 2) better resistivity
* Dedicated to Th. Dobzhansky on the occasion of
his sixtieth birthday.
291
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
292
RENSCH
its relatively smaller surface area, than to have
too large antlers as in Megaceros. Hence, if we
evaluate the functions of an animal as a whole
the giants may be called adaptive in their geological period. But when the climate became
successively warmer later on, the advantage of
giant size became reduced and the disadvantage
of the excessive size became more apparent, and
finally selection could wipe out such a species.
By such a change of the selective value of different
characters correlated with one another, we may
perhaps explain the dying out of so many species
towards the end of the Pleistocene. This short
discussion of Cope's rule may be sufficient to
exemplify the implications connected with correlative effects of trends.
When we now try to summarize the multiplicity
of trends which we mention in the lines of descent,
we must confess that a classification is very
difficult. We can only distinguish between general
and more specific trends. The former are of much
greater interest for the understanding of evolution
as they govern the development of many branches
of the phylogeny of animals. However, by using
the term "general trend" we do not want to
characterize a consistent successive trend but the
sum total of many single trends, not dependent
on one another, but showing the same general
tendency because of their parallel selective advantages.
We have already briefly discussed one of these
general trends: Cope's rule, which is connected
with some other rules of proportion. Another
trend, which is rather common in the development
of new types of morphology concerns the multiplication of equal structures. This is exemplified
by the multiplication of oscula in sponges, of
pharynges in Turbellaria, of proglottids in tape
worms, of segments in annelids, chilopods, diplopods, of ommatidia in arthropods, of vertebrae of
some fishes, amphibians and reptiles, of parts of
the kidney of vertebrates, of fingers of polydactylous whales etc. Probably this trend is so
common because it is rather easy to multiply by
mutation a well functioning structure developed
by many processes of selection during a long
phase of phylogeny, and to harmonize such a
multiplication with a normal functioning of the
body. It seems to be more difficult to develop
quite a new structure which would correspond
with a larger size of the body.
Rather often the trend for multiplication of
homonymous structures has been followed by
trend for growing heteronomy. This is exemplified
by the differentiation of the first segments of
Polychaeta, Crustacea anal insects or by the
development of legs into mouth-organs in Crustacea and insects, or by the growing heterodonty
of vertebrate teeth etc. In these cases which
correspond only in regard to their general
tendency, the advantage of division of labor was
decisive for the development of such trends.
The most general trend is probably the trend
for continuous alteration, that is to say, for continuous evolution and adaptation in consequence
of continuous mutation and selection.
Further, one of the most important general
trends is the evolutionary progress which we may
state in so many lines of descent and which also
caused the development of man. Evolutionary
progress is characterized by one or several of the
following factors (compare J. Huxley 1942, B.
Rensch 1943, 1954, 1959, G. G. Simpson 1949,
1953): 1) increase of complication; advantage:
increase of general efficiency and possibility of
division of labor; 2) increase of more rational
structures and functions; for example, by centralization of different functions; 3) special increase
of plasticity of structures and functions; for
example, genetical or anatomical plasticity,
accommodation, etc.; 4) increase of complication
and of division of labor in the central nervous
systems, that means structures which are capable
of especially plastic functions; 5) increase of
independence of changes in the habitat; 6) increase
of autonomy as a consequence of growing complication, rationalization, and plasticity. These
factors of evolutionary progress are only restricted in so far as they can only contribute to
definite progress. Hence, evolutionary progress
does not mean improvement of a species in its
special habitat (adaptiogenesis) but improvement
with regard to the further development of a whole
line of descent. We may distinguish this type
of progress as beltiogenesis (~ehv~c0v = better).
SPECIAL TRENDS TOWARDS EVOLUTIONARY
PROGRESS IN SENSE ORGANS AND BRAINS
In numerous special lines of descent which we
know sufficiently by paleontological, anatomical,
and embryological investigations, we often note
a progressive improvement of the sense organs and
nearly always of the central nervous system.
This is especially striking in the first period of
phylogeny when a new type of morphology has
been developed and successive improvement
takes place. Towards the end of a line of descent
this improvement slackens down and at last stops,
that is to say, because of the continuous operating
forces of selection, a more or less favorable state
will be stabilized. Thus, sense organs and central
nervous systems follow the well known rule of
decreasing speed of evolution, a process which
J. Huxley (1957) has called stasigenesis.
However, sense organs and central nervous
systems have a rather different speed of evolution.
Normally, the former ones show a quick improvement in the beginning of a line of descent and
then remain constant for a long period. The eyes
of vertebrates, for example, apparently developed
quickly into a type of vesicular eyes with lens,
iris, and complicated retina. The further improvement towards warm-blooded vertebrates
was brought about by increasing the number of
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
PROGRESS OF BRAINS AND SENSE ORGANS
sense cells and improving accommodation and
adaptation. In shorter lines of descent showing
much progress in other organs including brain
progress as in the horse line, or in the lines of
descent of carnivores and primates, the eyes
apparently remained nearly stable. Or, the number
of sense cells was increased because of the growing
size of the eyes which we may estimate from the
growing size of the orbitae.
The same holds good for all other sense organs.
In primitive mammals the labyrinth, nose, tongue,
the organs for the sense of touch, etc., are well
developed and in most cases higher mammals
show only a rather insignificant improvement by
increase of the number of sense cells. This increase
normally corresponds to the increase of body
size. For example: the small newt Triturus vulgaris
has about 171,300 sense cells in each eye (diameter
2.2 mm.), while the larger Tr. cristatus (diameter
of eye 2.7 ram.) has 224,300. The small salamander Salamandra atra (diameter of eye 3.5
mm.) has about 386,300 sense cells whereas the
larger S. maculosa (diameter of eye 4.8 ram.)
has about 533,000 (A. M511er 1950).
The central nervous systems, on the other
hand, show a steadier improvement in the lines
of descent. While eyes, labyrinth, etc., reached
a high stage of development on the level of
reptiles, the brain continued to improve continuously from reptiles to lower mammals and
further on to higher mammals and to man.
Among fish, the forebrain is more or less a mere
smelling center. In reptiles, it is relatively much
larger and has several new sensory and associative functions. In mammals, the cortex was
added as a new superimposed region of great
complication and in several higher orders of
this class new regions of association, especially
in the front region, were added.
Even in special lines of descent, as for example
in the previously noted lines of horses or primates
we may see a striking progress. In the line Eohippus-Mesohippus-Merychippus-Equus or in the
lines leading from Thinocyon (Creodonta) to later
genera of carnivores, T. Edinger (1948, 1956), by
preparing casts of the brain case, showed that the
forebrain became not only absolutely, but also
relatively larger, and that the cortex increased
very conspicuously by folding. The evolution
from ape to man was also accompanied by marked
alterations of the forebrain especially by an
increase of the associative regions of the frontal
and partial increase of the temporal lobes. Judging
from casts of the brain case, the Australopithecinae had a frontal region which was only slightly
larger than that of a chimpanzee. It is doubtful
if a motor region of speech (Broca's region) in
these ape men began to develop (compare Fig. 1,
and W. G. H. Schepers in Broom and Schepers
1946). In Sinanthropus and in Homo neanderthalensis, the frontal lobe was much more developed.
The existence of Broca's region is very probable
293
FIGURE 1. Lateral view of forebrains. Equally diminished in size. Above: chimpanzee, middle : reconstruction
(after cast of brain-case) of Plesianthropus (Australopithecus), below: of a negro. Motor praecentral region
black, frontal brain (mainly associative centers) dotted.
Broca'region (and possible prestage in Plesianthropus)
densely dotted. (After G. W. H. Schepers from B.
Rensch 1959a).
but the basal cortex of the frontal lobe was only
slightly folded compared with the structure in
Homo sapiens (H. Spatz 1950). Judging by
findings of pathologists, this basal region is
especially important for the consistent series of
hypotheses and for creative performances of man
(K. Kleist 1934).
We may also state different trends with regard
to the histological differentiation and division of
labor among different regions in the forebrain of
vertebrates. The undifferentiated dorsal brain
tissue of Agnatha became a tripartite pallium in
fish and amphibians, the lateral regions of which
were changed into the neocortex of mammals.
During the phylogeny of mammals, the relative
extension of the isocortex, that is to say, of the
most complicated 5- and 7-layered cortex was
enlarged and divided into more and more special
functional regions.
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
294
RENSCH
Parallel to this development, a growing number
of special cells originated. Besides, the pyramidal,
granular, and star cells already known in Anamnia
and the giant star cells already existing in reptiles,
later, compass and bifurcated cells arose in mammals (still lacking in Insectivora; A. Syring 1957).
Apparently most of these types of cells have the
function of integrating excitations in a special
manner.
Will it now be possible to analyze these different
trends of progression in vertebrates by quantitative statements? This seems to be the case although normally we cannot study the lines of
descent themselves. As already mentioned, there
are only a few lines like that of the horse where
we may estimate the alteration of the superficial
brain structure, especially of portions of different
parts and of the main foldings from casts of the
brain case. However, we may use a series of brains
of recent species as models of lines of descent. We
may compare the absolute and relative size, and
thus the progress of the whole brain, of the forebrain or of single regions in the "ascending line"
of vertebrates or of mammals only; for instance of
Monotremata, Marsupialia, Insectivora, Carnivora, and Primates. Of course, this method is
not quite correct. One may object that the socalled primitive types of recent species may be
primitive only in some characters of the brain and
that other characters may be rather progressive.
There is no doubt that this is the case in many
species. The Monotremata, for example, have a
relatively large forebrain and the kangaroos show
a fair learning capacity (see below).
However, if we restrict our conclusions to
some quantitative relations this method of comparing series of recent animals can be correct.
If we are capable of establishing general rules
valid for many related species differing only in
absolute body size or absolute brain size, then we
must assume that these rules were already valid
in the common ancestors of the compared species
and hence also in the lines of descent of large
species normally beginning with smaller species
(Cope's rule).
By such comparisons it has been possible to
note a certain relation between brain size (E) and
body size (C) which may be expressed by the
formula E = b 9C a. In this function formula, the
allometrical exponent a indicates the relative
growth of the brain in series of increasing body
size.
For mammals, different but similar values have
been calculated. By comparing many species of
different orders E. Dubois (1898, 1930), L.
Lapique (1898, 1907), and R. Brummelkamp
(1946) found a = 0.56, D. P. Quiring (1938)
found 0.58 for African ungulates, G. yon Bonin
(1937) who investigated many species of different
orders of mammals found a = 0.66, H. J. Jerison
(1955) a = 0.67, and O. Snell (1891) a = 0.68.
Apparently these differences depend on differences
of preparation and weighing, and also on differences between carnivorous and herbivorous, or
between quicker and slower, or burrowing and
free-living, species. Hence, D. Sholl found statistically significant differences between two
families of rodents: in Sciuridae he calculated a
= 0.60; in Muridae a = 0.51. But in spite of such
differences we can say that in mammals, the
phylogenetical increase in body size is accompanied by an increase of the brain which is more
or less proportionate to the surface area of the
animals (a = 0.60). This rule seems to be related
to the fact that the innervation of the epidermis
and many interior organs depend upon the surface
areas.
Considering these allometrical exponents it
becomes possible also to find a formula for the
relation between brain and body size or weight,
indicating approximately the level of phylogenetical progress of a brain. Perhaps the best formula
was developed by R. Mfiller (based on E = b.
C~
brain index
brain weight3
- body weightv In another
manner K. Wirz (1950) calculated the level of
"cerebralization" by a "neopallium index", which
does not satisfactorily reflect the level of progress
in all cases. This author divides the brain
weights by an artificial basic figure which would
indicate the weight of the brain stem if the
animal in question belonged to the Insectivora.
However one may object that the brain of ancestral species of ungulates, carnivores, and other
groups did not correspond to the brains of recent
Insectivora in the Early Tertiary, but showed a
more primitive, reptile-like structure.
Of course, the absolute and relative size of the
brain or of the forebrain is not yet a sufficient
criterion for the phylogenetical level. The histological structure, too, is of great importance. We
now know that in mammals the density of cortex
cells decreases with increasing absolute brain
size. S. T. Bok and M. J. Van Erp Taalman Kip
(1939) could show this in rodents of different body
size, and D. B. Tower (1954) described it in
mammals of other orders from mouse to elephant.
The latter author stated that the decrease can be
expressed by the equation N = KN 9W R in which
N indicates the number of nerve cells per unit of
volume; KN means a constant; W, the brain
weight; and R, the coefficient of regression of the
species in question. Corresponding to this rule
the activity of the acetylcholine system which is
parallel to the volume of nerve cells shows the
same relation to the brain size. On the other hand,
larger species of vertebrates nearly always have
larger neurons showing a much richer ramification
than related smaller species (S. T. Bok 1936, B.
Rensch 1949, A. Spina Franca Netto 1951).
Hence, in absolutely larger brains more possibilities of connections of fibers and associations
exist which is an advantage and which may
contribute to brain progress.
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
PROGRESS OF BRAINS AND SENSE ORGANS
295
TABLE 1. RELATIVE SIZE OF THE SURFACE (IN PER CENT
OF THE SURFACE OF THE WHOLE FOREBRAIN) OF 4
DIFFERENT REGIONS OF THE CORTEX OF WHITE RAT
AND WHITE MOUSE AND OF GIANT SQUIRREL ( R a l u f a )
51
""q"
,w
~o 2'0 io
,~o go 6o
7b sb
9'o,~,~
AND DWARF SQUIRREL (Funambulus)
In parentheses: number of analyzed hemispheres.
(After K. W. Harde and CH. Schulz).
Muridae
39
38
35J
~
.
.
Io
z0
(12) white (6) white (3) Ratufa (4) Fur~indica
ambulus
rats
mice
palmarum
...........
""'l"
i0
~,o
i0
6o 7b
Sciaridae
8o 4 0 d ~
FIGUHE 2. Changing relative size of the sehizocortex
(above) and the holocortex 7-stratificatus (below) in
per cent of the whole surface area of the hemisphere
(ordinate) during the postnatal ontogeny of the white
mouse. Wedge = day of the opening of eyes, hatched
part = beginning maturity (after K. W. Harde).
Still more important is the fact that the cytoarchitectonieal structure of the forebrain is altered
corresponding to increasing brain size. This alteration is advantageous. Of course, in this case, we
can only estimate phylogenetical alterations by
comparing related recent species of different
body size and brain size. However, as already
mentioned, we are entitled to use this method.
The author has repeatedly summarized (last
in 1958) these investigations carried out in the
Zoological Institute of the University of MSnster
for many years. Hence, I may restrict myself to a
short outline of the main results with regard to
the cortex of mammals. By measuring a large
number of sections of the forebrain of mice, K. W.
Harde (1949) could show that most regions and
smaller areas grow in a different manner with
positive or negative allometry in relation to the
whole cortex. In some stages a change in the
direction of allometry takes place, for instance,
after birth, at the period of the opening of the
eyes, or at the beginning of maturity (Fig. 2).
Hence, when allometrieal exponents remain
more or less constant during phylogeny, related
species of different body size show different
proportions of cortex regions. In some eases the
single allometrieal exponents have been altered
markedly during phylogeny apparently in consequence of special selection caused by special
modes of life. But in most eases the general
allometrical tendency remained the same and
only the value of the allometrical exponents increased or decreased but did not change from
positive to negative allometry. And especially
one very important tendency remained unchanged: the relative enlargement of the isoeortex,
that is to say, of the most complicated and most
progressive 5- and 7-layered regions. This enlargement took place partly at the cost of phylogenetically older regions, like the semicortex or schizo-
Average weight of animal..
214 g
26 g
1237 g
111 g
Isocortex
Bicortex
Schizocortex
Semicortex
53.6
23.1
9.6
13.9
49.4
22.9
9.0
18.7
70.5
14.4
6.3
9.2
62.4
18.2
6.6
12.8
J
TABLE 2. RELATIVE SIZE OF THE SURFACE (IN PER CENT
OF THE SURFACE OF THE WHOLE FOREBRAIN) OF 4
DIFFERENT REGIONS OF THE CORTEX OF 4 BATS OF
DECREASING BODY SIZE
(After F. Ltitgemeier)
Pteropu
medius
aulti
Average weight of animal..
723 g
85g
Isocortex
Bicortex
Schizocortex
Semieortex
63.9
18.4
5.0
12.7
53.8
23.3
6.0
16.9
serotinus daubentoni
/_ 20g
93.8
34.2
9.7
16.3
8,5g
37.4
33.6
12.9
16.1
cortex. Tables 1 and 2 show these differences of
relative size in rodents (K. W. Harde 1949) and
in bats (F. Luetgemeier). As this rule seems to be
valid for species of different body size in 4 families
of 2 different orders of mammals and as it also
proves right in Primates (if we compare monkeys,
apes, and man), and in mammals as a whole, (if
we compare more primitive with more progressive
orders), we may assume that we have to do with a
general rule. It may be explained by the greater
selection value of the more complicated cortex.
This histological complication allows more complicated connections of nerve fibers, more complicated associations, a richer memory, and hence
a more plastic behavior which is better adapted to
different external situations.
Summing up, we may agree that a progressive
trend in the evolution of the brains of vertebrates is characterized by the following advantages: 1) increase of absolute brain size, in longer
lines of descent and increase of relative brain
size; 2) increase of the histological complication
and of the division of labor of the forebrain,
which finally produced the multilayered cortex
of mammals; 4) increase of the 5- and 7-layered
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
296
RENSCH
isocortex; and 5) of the special regions of association in mammals.
In this context we must bear in mind that by
the increase of the number of neurons the possibilities of associative connections will increase in a
geometrical progression. Between 4 neurons, 6
different possibilities of mutual connections exist,
between 8 neurons, 26, and between 16 neurons,
120. Of course, it will be impossible for each
neuron to come in contact with each other one
but this fact does not alter the general statement
that the possibility of connections and the degree
of functional capabilities will increase very
markedly by each increase in number of neurons.
However, only in the course of long lines of
descent, the progressive increase of the brain of
vertebrates was accompanied by a noticeable
increase of the number of neurons. This is seen in
the lines of descent leading from the Creodonta
of the Eocene to recent Carnivora, from lemurs
to monkeys, apes and man, and in the lines
leading from the Insectivora of the Cretaceous
to all higher orders of placental mammals. In
shorter lines of descent, the number of neurons
remains more or less the same because, as already
mentioned, the density of cells decreases along
with the increasing brain size. However, parallel
with the increase of cell size, the number of
dendritic ramifications will be increased (S. T.
Bok 1936, B. Rensch 1949, G. A. Shariff 1953, A.
Spina Franca Netto 1951), and therefore the
capability of more complicated connections and
associations will increase very markedly.
Now, it is an important task to find out in which
manner better nervous functions run parallel
with the increase of brain size or size of the
forebrain and also with the increase of histological
differentiation, especially with the increase of the
5- and 7-layered isocortex. There can be no doubt
that the brain functions of higher mammals such
as carnivores, monkeys, and apes having an
absolutely large folded forebrain, show a much
greater multiplicity than the brain functions of
lower mammals with unfolded forebrain as, for
example, rodents and marsupials. Correspondingly
the brain functions of birds show a much greater
multiplicity than the brain functions of reptiles or
amphibians. However, fish have a small brain
and a forebrain which functions only as a center of
smell, but show much better capabilities than
amphibians and perhaps some reptiles, too, although the latter have a relatively large forebrain. Trout, which were used in the experiments
of A. Saxena who worked in our Institute, were
capable of learning 6 visual tasks, discrimination
between two colors or two patterns in black and
white; they could solve these problems in multiple
tests (Table 3). Such fish were able to retain a pair
of patterns of cross against circle more than 80
days, and two colors (red against green) more
than 150 days. There are no reports of similar
capabilities of amphibians and reotiles. Lizards
T A B L E 3. RESULTS OF A M U L T I P L E T E S T
IN
TROUT
Percentage of correct choices of 3 specimens having
learned 6 visual tasks. Average of 60 trials for each task
for each fish. (After A. Saxena).
Tasks
1
Trout 1
Trout 2
Trout3
2
3
4
5
6
83
81
78
81
81
88
78
86
81
83
81
81
[ 86 I 85 I 83 / 86 I ~ I 90
trained by H. Wagner (1933) and by H. Ehrenhardt (1937) were not able to learn differences
of colors or patterns in black and white. (C.
Hinsberg, working in our Institute, seems to get
better success in training young iguanas and
turtles.) R. J. Wojtusiak (1933, 1934) succeeded
in training turtles to discriminate between pairs
of colors and geometrical patterns. Hence, we
may doubt that a purely quantitative increase of
the brain or the number of neurons may raise
the learning capability.
However, a correlation between brain size and
learning capability becomes very probable from
other experiments which we performed in the
Zoological Institute of Mfinster during recent
years. We trained related species of markedly
differing body size and hence very different brain
size, to learn the same tasks in order to find out
whether or not the larger species or races showed
better capabilities. We could state that generally
the learning capacity of larger species was better
than the capacity of related smaller species. White
rats were able to learn successively 8 visual tasks
which they mastered in multiple tests whereas
white mice mastered only 6 tasks (W. Reetz
1957). An Indian elephant mastered 20 visual
tasks at the same time (B. Renseh and R. Altevogt 1955), a horse, 20 tasks, a donkey, only 13,
and a zebra, only 10 (H. D. Giebel 1958). A large
race of domestic fowl, the brahmas, mastered 7
pairs of patterns in multiple tests, medium-sized
races, only 5, a dwarf race, 4 to 5 (R. Altevogt
1951). As already mentioned, trout (Trutta shasta)
mastered 6 similar visual tasks in multiple tests
(A. Saxena), small Cyprinodontidae (Lebistes and
Xiphophorus), only 2 or perhaps 4, and not in a
statistically significant percentage (B. Rensch
1954b).
We found corresponding differences of larger
and smaller species or races when we tested the
time of retention. The best of the rats trained by
W. Reetz solved one of 8 learned tasks after
459 days, whereas the best of her mice retained
one of 6 learned tasks only 195 days. The giant
race of fowl of R. Altevogt solved all the 6 trained
tasks after an interval of 20 days in a statistically
sufficient percentage, while the dwarf race retained
only 3 to 5 tasks. The trout of A. Saxena mastered
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
PROGRESS OF BRAINS AND SENSE ORGANS
a single visual task (two black and white patterns)
after 80 days, another task (colors) after 150
days without training. The larger Cyprinodontidae which I had trained myself retained a visual
task for about 54 days on the average.
It is not certain that larger animals have a
better capability of recognizing a learned pattern
when the size or the color is altered or when the
pattern is transformed in such a manner that only
partial components remained the same or that
only a relation between two patterns (for example
larger-smaller) remained unchanged. In this
respect rats are apparently a little better than
mice, giant races of fowl better than dwarf races
(experiments of W. Stichmann under way in our
Institute), trout better than Cyprinodontidae.
However, the results are not yet sufficient for a
generalization to be made.
Summing up, we may establish a working
hypothesis based on the following facts: larger
vertebrates have a greater learning capacity and
a longer capability of retaining visual tasks than
related smaller species or races with absolutely
smaller brain, having a similar mode of life. There
can be no doubt that these capabilities of larger
animals (larger brains) have a positive selection
value, which has determined the tendency of
evolutionary development. It is rather probable
that it was, at least partially, a victory of the
better brains when, in former geological epochs,
the Agnatha were replaced by competing higher
types of fishes, marsupials by placental mammals,
Creodonta by carnivores, and lemurs by monkeys.
In many lines of descent natural selection caused
an increasing enlargement and improvement of
the brains and therefore it favored the increase of
the correlated body size, especially by interspeeific
competition for food and habitat. Thus it contributed to formulation of Cope's rule.
SPECIAL PROGRESSIVE TRENDS IN SENSE ORGANS
AND BRAINS OF INSECTS
In order to generalize upon the results found in
vertebrates, it is necessary to carry out corresponding investigations in invertebrates, especially in groups showing a complicated brain. It
became obvious therefore that differences in the
brain of related larger and smaller species of
insects should be analyzed and that the allometrical tendencies during ontogeny should be studied.
In this case we must bear in mind that we
cannot compare actual lines of descent, and that
the size of the brain capsule of fossil forms does
not indicate the brain size because the size of the
head depends to a large degree upon the relative
size and special organization of the mouth parts
and of the eyes and antennae. Hence, the brain
capsule may be filled out with brain to a very
different degree as modern species will show.
However, we may consider the recent species of
the order of Blattaria, an order existing since the
Carboniferous, as representatives of a primitive
297
type, and the species of Hymenoptera, Coleoptera,
Diptera, etc.--that is to say of orders developed
in the Mesozoic--as more advanced types.
Furthermore we have to consider that a phylogenetical increase of body size did not take place
(or did so only to a small extent) in insects. As
these animals have an external skeleton, large
species had to develop an excessively thick
skeleton which would cause a negative selective
value. On the other hand, the capability of flying
also restricts a possible increase in body size, for
the body weight grows by the third power whereas
the surface area and hence the function of the
wings grows by the second power. However, in
spite of these restrictions we can state certain
rules of correlation between body size or brain
size and special brain structure, if we compare
related species of a similar mode of life. And these
rules show several parallelisms with the rules of
vertebrates.
We may also obtain corresponding results if
we compare the sense organs of smaller and larger
species. This becomes especially conspicuous by a
comparison of the number of ommatidia. A few
examples may be sufficient. The tiny beetle,
Bryaxis haematica (body length 2 mm.), has only
32 ommatidia whereas the giant beetle Archon
centaurus has 29,450 (K. Leinemann 1904). Drosophila melanogaster ( 9 ) has about 668 ommatidia;
the large fly Calliphora erythrocephala ( 9 ) 4,651
on the average; the tiny gallmidge Contarinia
(o~) 140; the much larger Culex pipiens (c~) 488;
the still larger Tipula oleracea ( 9 ) 1,848 on the
average (W. Partmann 1948). The numbers of
smelling cones and of sensitive bristles on the
feelers of comparable larger and smaller species
show similar large differences (for example Melotontha compared with Phytlopertha or Geotrupes
compared with the small species of Aphodius).
If we now compare the relative size of the more
important structures of the protocerebrum we
may see certain trends which are correlated with
growing or decreasing body size. Detailed investigations of the brains of many species of Blattaria,
Hymenoptera, Diptera, and Coleoptera (H. Goossen 1949, R. Neder, W. Hinke, E. Bertram),
carried out in our Institute, show that larger
species generally have relatively larger and more
differentiated corpora pedunculata in which the
surface of the neuropil is more folded. In a similar
manner as in vertebrates it is the most anterior
part of the brain which is mainly enlarged. The
corpora pedunculata may be looked at as the
histological basis of many instincts and of learning
processes because they are much more developed
in insects with complicated instincts, like the
social Hymenoptera. But contrary to the differenees among vertebrates, the enlargement is not
caused by an enlargement of the neurons but by
an increase of the number of neurons, in the ease
of the so-called globuli cells (Table 4). However,
the central body, the lobes and the pons of the
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
298
RENSCH
neurons in insects has been caused b y the rich
development of sense organs, especially of eyes
and of organs of smell. Thus, m u c h more complex
patterns of excitation arose in the sense organs
and these patterns were favorable only if they
could be transferred in undiminished complexity
to the brain. This was possible only by increasing
the n u m b e r of neurons there. I n this manner, the
brain could respond to single details of a pattern
of excitation. As the size of an insect is limited by
the tolerable size of the head and as there arose
an enormous number of sense organs, the neurons
of the brain had to become very small. We can
find the same tendency of decrease of sensorial
cells in the brain in cephalopods and in vertebrates, t h a t is to say, in groups of animals showing
a similar increase and improvement of sense
organs.
Of course, the growth ratios determining the
differences in the brain of imagos of large and
small species are dependent to a large extent
upon the differences of the larval development.
I n Blattidae, t h a t is to say in a primitive hemimetabolic group, R. Neder, working in our Institute,
stated t h a t the corpora pedunculata grow with
positive allometry up to the third larval stage,
later on up to the last stage (the 6th in Phyllodromia, the 10th in Periplaneta) or to the imago
with negative allometry (Fig. 3). However, in
holometabolic insects, the larvae of which differ
very much from the imagos, the corpora pedunculata grow with positive allometry during the
whole larval development (Apis: E. Bertram), or
protocerebrum and the lobi optici are relatively
smaller in large species.
I t is of great interest t h a t the Polychaeta show
quite different tendencies. R. B. Clark (1957), who
investigated the brains of 17 species of very
different body size in the genus Nephtys (5 to 300
m m . long), stated t h a t the number of neurons
was always the same, whereas the size of the
neurons was different in proportion to brain size.
Hence, the totally different tendency of insects
was a new phylogenetical acquisition. Probably
the new trend of increasing the number of brain
TABLE 4. NUMBER OF NUCLEI OF GLOBULI CELLS IN A
SECTION (THICKNESS 10 p) THROUGHTHE CENTER
Of CORPORA PEDUNCULATA
Differences of related species of different body size.
(After H. Goossen).
Body length
in mm.
Carabidae
10phonug pubescens c~
1 Agonum muelleri 9
Dytiscidae
1 Dytiscus marginalis Q
1 Ilybius fenestratus c~
,~carabaeidae
4 Geotrupes stercorarius
1 Aphodius fimetarius
1 Melolontha vulgaris o~
1 Phyllopertha hosticola
Number of
nuclei
11.6
6.2
614
208
27.0
12.0
615
188
17.3
6.5
20.6
9.9
247
101
1776
624
%
3~
o
32
30
28
26
2~
22
20
18
16
12
I0
I
I
I
I
I
I
I
0.I
0,2
0.3
0.~
05
0,6
0.7
I
0.8 mm3
FIGURE 3. Volume of the corpora pedunculata in percent of the volume of the whole brain (ordinate) in growing
cockroaches. Crosses ( 9 ) and stars (~) = Periplaneta americana, white ( 9 ) and black (o~) circles = Blatta orientalis,
white (9) and black ((~) triangles = Phyllodromia germanica. (After R. Neder).
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
PROGRESS OF BRAINS AND SENSE ORGANS
they grow with negative allometry nearly during
the whole larval period. Only shortly before the
development of the pupa in Culex or during the
early pupal period in Drosophila they grow with
strong positive allometry (Fig. 4; W. Hinke). As
there is almost no further growth after the pupal
period, it was necessary to chart the number of
hours on the abscissa in Fig. 4. Hence, the graph
shows that after quick growth of the corpora
peduneulata in the pupal period, the definite
portions of the brain are developed and remain
constant. In Myrmeleo, an insect which has a
rather active larva, the corpora peduneulata are
developed rather early. The central bodies, on the
other hand, grow with strong positive allometry
since the third larval stage in bees, or after the
development of the pupa in Culex and Drosophila.
In the larvae of Drosophila, we may also estimate the correlations between the corpora peduneulata and the lobi optici by comparing the larval
development of mutants with reduced eyes (Bar
%
u\
',n
',
'..,.
"..
"-.
A~
m
~o
w
9
"
9
,A . . . . . . . . . . .
A . ~ _-.-, . _ . A
..u ....
,i
s~ ,
[.,'
~..' 9
FIGURE4. Above: relative growth of the corpora peduneulata; below of the lobi optiei in per cent of the brain
in 3 strains of Drosophila melanogaster. Abscissa: hours
of development and stages of larvae, pupa and hatched
imagos. IH and II H = 1. and 2. moult of the larva,
AL = old larva, V = time of pupation, KA = turning
out of the head, Sehl. = hatching.
----wild
-
-
- B a r
..... eyeless
299
and eyeless) with normal flies. Such investigations
are of special interest because they show the
developmental effect of a single mutation. Thus,
we may analyze one of the chains of reaction
which are controlled by the allele in question.
Besides, the correlations which we may observe
in such a manner show some of the pleiotropic
effects. One of my students, W. Hinke (not yet
published) showed that in such mutants the lobi
optici are not developed to a normal extent during
the pupal period. This alteration also affects the
development of the corpora peduneulata. The
mutant Bar shows a marked decrease of the
positively allometrieal phase of the corpora peduneulata. In the early pupal period and in the mutant eyeless, the corpora peduneulata grow only
isometrically in this period (Fig. 4).
With regard to these differences in the main
developmental phase of the corpora peduneulata
as seen to exist between hemimetabolic and holometabolic insects, it becomes conceivable that
giant species of both groups show different
tendencies of the brain. The Australian giant
cockroach Maeropanesthia rhinocerus does not
show, contrary to the above mentioned rule of
body size, an increase in the relative size of the
corpora peduneulata compared with Periplaneta
americana, the body of which has only ~ 2 the
weight of Macropanesthia. Apparently the phylogenetic development of this giant species took
place through prolongation of the last larval
phase or by addition of new larval stages at the
end of the phase which shows a slightly negative
allometry of the corpora pedunculata. It is a
pity that the ontogenetieal development has not
yet been studied.
C. Ratzendorfer (1952) tried to determine the
levels of progress of insect brains in a similar manner as in vertebrates, that is to say, she calculated
the higher differentiated parts of the brain as per
cent of the less specialized remainder of brain
parts. This corresponds more or less with the
usual method of characterizing the phylogenetieal
level by the relative size of the corpora pedunculata. Thus, Ratzendorfer could show that progressive brain types may be found anaong holometabolie as well as among hemimetabolie insects.
Unfortunately, differences between larger and
smaller related species of insects with regard to
the capacity of learning or retaining have not yet
been carried out sufficiently. Extensive experiments with Carabidae carried out by the author,
showed a better capability of retention for some
larger species, but the results were rather mixed
in kind. However, it is probable that the abundance
and the complication of instincts are much greater
in larger insects (that means in insects with
absolutely larger brains) than in smaller related
species. All large species of European bees and
wasps, for example, show very complicated social
instincts, whereas the small species of the same
families are solitary. The Searabaeidae show
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
300
RENSCH
corresponding differences. Scarabaeus, Copris, Geotrupes and other large species have much more
complicated instincts for care of eggs than related
small species of the genus Aphodius. Among
Silphidae, Necrophorus has much more complicated instincts for the care for eggs and larvae
than the small Catops.
GENERAL CONCLUSIONS
From the investigations reported above we
may conclude that a purely quantitative increase
of the structure of brains and sense organs can
produce evolutionary progress especially by increase of better central nervous and parallel
psychic performances. Plus mutations causing
such a growing intensity or a prolongation of the
growth ratios of sense or brains are always possible
and are rather common. They can be advantageous because an increase of sense cells or brain
neurons enables an animal to react to finer
details of a pattern of excitation. When in primitive Arthropoda, compound eyes began to develop
during phylogeny they were able to discriminate
only between dark and light and between directions of light, as long as the number of ommatidia
remained small (as in Cladocera and Copepoda).
Only a strong increase in the number of ommatidia, as this is exemplified by higher Crustacea,
allowed the discrimination of details of a pattern
of excitation and thus discrimination of details
of a pattern of excitation and thus a visible
reaction to prey, to enemies, etc. The same
holds good for the neurons of the brain, the
increase of which enables an animal to answer to
complicated patterns of excitation in more detail,
to develop more plastic reactions and a more
complicated memory. As all such alterations had
a positive selective value they automatically
caused an evolutionary progress in many lines
of descent. In most cases a correlation between
increase of brain size and increase of body size
existed (Cope's rule) and the latter was often
advantageous, too.
If now a new additional brain region arose by
mutative increase of the positive allometry of a
neighboring region, a new combination of excitation flowing in from all neighboring regions took
place. In such a manner new functions could
arise and a special selection of these new functions
could begin. A typical example for such a development of a new level of integration is the evolution
of the motor center of speech in human brains,
of Broca's region. Here we have an additional
region of the lateral frontal brain which neighbors
the regions for tone memory, for the movements
of mouth and tongue, and for the motor impulse
of complicated associations centered in the basal
part of the frontal lobes. Hence the newly developed region was more or less predetermined for
becoming a motor center for speaking. However,
as soon as the new function, speech, began to
develop this new character of man became so
important, because of the possibility of more
abstract thinking, the mutual exchange of experience, and the development of tradition, that now
a special selection quickly favored the further
evolution of Broca's region.
Similar additions of brain regions primarily
without a special function seem to have occurred
several times during the evolution of the brain of
recent mammals. They had a positive selection
value because they allowed an increasing division
of labor. As now the main functions of the brain,
especially the well differentiated centers of sense,
existed on more primitive phylogenetical levels,
normally the larger additional regions could only
function as co-ordinating and integrating centers
on a higher level. The basic evolutionary process,
however, was only the appearance of plus mutations favoring certain growth ratios and secondarily allowing new processes of special selection.
A few examples may illustrate this.
The forebrain of vertebrates became relatively
larger in the course of evolution. In amphibians
and still more in reptiles it became the largest
part of the brain. Its functions, however, still
remained rather restricted. The learning capacity
and the memory of amphibians and even of
reptiles is very limited although the latter show
more histological differentiation which includes a
visual center. In mammals, the forebrain becomes
the dominating brain part as it co-ordinates
many excitations of other parts. In the forebrain,
the cortex becomes more and more complicated
and gains the function of co-ordination and finer
evaluation of subcortical excitations. In lower
mammals, the whole cortex is divided in several
fields of projection for different sense regions.
When later, the cortex became still more enlarged, the new regions, especially in the front
part and in the temporal lobes, could assume
again a new directing function: they became
regions of association in which the excitations of
the same regions were combined and evaluated on
a higher level. Finally, during the evolution of the
genus Homo, Broca's region of speech was developed which facilitated not only speaking but
also thinking in words in a more abstract manner,
as mentioned above. On the other hand, the
basal part of the frontal brain became more and
more folded (in Pithecanthropus it was unfolded:
H. Spatz 1950) and here a region arose acting
as a kind of impulse-giving motor for complicated
associative thinking. The development of these
typically human brains probably based on purely
quantitative mutations, was perhaps the decisive
event in the origin of Homo sapiens. It may be
that the origin of these regions stopped the rapid
increase of the brain, because the accumulation
of tradition replaced the selection pressure for
pure increase in size.
All the mentioned examples show that a new
region, developed by enlargement of neighboring
regions, may secondarily get new functions. In
such cases we may speak of a postintrogression of
functions.
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
PROGRESS OF BRAINS AND SENSE ORGANS
Summing up, we m a y state once again that the
improvement of sense organs and of brains which
is characteristic of the evolutionary progress, may
be explained by considering only quantitative
basic alterations, that is to say undirected plus
mutations which determine the growth ratios and
which m a y be favored by selection. Such quantitative alterations did not only increase the corresponding functions but also caused the development of quite new levels of integration. Because
of the systemic laws of correlation, this integration
m a y also lead to new qualitative characters.
As quantitative plus mutations m a y arise at
at any time and as advantageous varieties are
favored by selection (favorable structures of
brain, especially by interspecific selection, for
example: placental animals competing with marsupials), a progress of brain structure in different
lines of descent must result. Here we touch on
the important question of how far the origin of
man was necessitated. In our context it will be
impossible to treat this special problem and I will
refer only to the corresponding short discussion in
m y little book on H o m o s a p i e n s (1959a).
On the psychic side, new levels of integration
have been developed successively. However, they
do not run parallel morphological levels as closely
as one could expect. As already mentioned above,
the morphological dominance of the forebrain in
amphibians and even in some reptiles did not
produce better performances than shown by
fishes, and the development of a cortex in lower
mammals likewise did not produce better performances than those of birds. This lack of parallelism
became evident in the learning capacity of different classes of vertebrates. M y coworkers always
used the same or very similar patterns for training
different animals. An opossum was capable of
learning one pair in a pattern, a giant kangaroo,
however, mastered six pairs in a pattern in a
multiple test (Neumann), white mice mastered
six pairs, rats 8 pairs, a donkey 13, a horse and an
elephant 20. Domestic fowl without true cortical
regions also learned 6 to 7 similar tasks, and even
trout, working only with the midbrain, mastered
6 tasks in a multiple test. Hence, I have the impression that the learning capability and the
psychic performances on the whole depend more
upon the number of brain neurons and on the
size of the associative brain parts than on the
histological differentiation, although the latter
is important, too.
The special psychic levels of integration leading
to the origin of man m a y perhaps be characterized
by the following succession: increase of free
choice based on experience (parallel: reduction of
pure instinctive performance); performances by
insight (foresight, thinking by including possible
future performances) knowledge of causal connections, abstract thinking by the help of words,
knowledge, and understanding of the laws of
the world. Wrong ways of evolution, that is to
say wrong thinking and acting has always been
301
corrected by natural selection. This means the
whole evolution of thinking and other psychic
processes also show a successive adaptation to the
laws of the world.
REFERENCES
ALTEVOGT,R. 1951. Vergleichend-psychologische Unter-
suchungen an H~ihnerrassen stark unterschiedener
KSrpergrSge. Z. f. Tier-Psychol. 8: 75-109.
BOK, S. T. 1936. The branching of the dendrites in the
cerebral cortex. Proc. Kon. Ak. Wetensch. Amsterdam 89: Nr. 10.
BoK, S. T. and VAN ERP TAALMANKIP, M. J. 1939.
The size of the body and the size and the number
of the nerve cells in the cerebral cortex. Acta Neerland. Morph. 3: 1-22.
BONIN, G. VON. 1937. Brain weight and body weight of
mammals. J. Gen. Psychol. 16: 379.
BROOM,H. A. and SCHnrERS, G. W. H. 1946. The SouthAfrican fossil ape-men. The Australopithecinae.
Transvaal Mus. Mere. 2, Pretoria.
BRUMMELKAMP, R. 1946. On the dependence of the
weight of the brain on the 2/9, resp. the 5/9 power
of the weight of the body. Kon. Nederl. Ak.
Wetensch. Proc. 49: Nr. 6.
CLA!~K,R. B. 1957. The influence of size on the structure
of the brain of Nephtys. Zool. Jahrb., Abt. allg.
Zool. 67: 261-282.
EmNGEa, T. 1948. Evolution of the horse brain. Geol.
Soc. America, Mere. 25, Baltimore.
1956. Objects et r6sultats de la pal~oneurologie. Ann.
de Paldont., $2: 97-116.
EHRENHARDT, I-I. 1937. Formsehen and Sehschgrfebestimmungen bei Eidechsen. Z. vergl. Physiol. 2~:
248.
DAY, M. F. 1950. The histology of a very large insect,
Macropanesthia rhinocerus Sauss. (Blattidae). Australian J. Sci. Research, set. B, Biol. Sci. 8: 61-75.
DuBms, E. 1898. l~ber die Abhiingigkeit des Hirngewichts yon der K6rpergr6ge. Arch. Anthropol.
(D) 25.
1930. Die phylogenetische Grolihirnzunahme, autonome Vervollkommnung der animalischen Funktionen. Biologia Genetica, 6: 247-292.
GIEBE5, H. D. 1958. Visuelles LernvermSgen bei Einhufern. Zoo[. Hahrb. Abt. allg. Zool. 67: 487-520.
GOOSSEN, H. 1949. Untersuchungen an Gehirnen verschieden groger, jeweils verwandter Coleopterenand Hymenopteren-Arten. Zool. Hahrb. Abt. allg.
Zool. 62: 1-64.
HARDE, K. W. 1949. Das postnatale Wachstum cytoarchitecktonischer Einheiten im Grofihirn der
weissen Maus. Zool. Jahrb. Abt. Anat. 70: 225-268.
I-IILTERMANN, I-I., U. KOCH, W. 1950. Taxonomic und
Vertikalverbreitung von Bolivinoides-Arten im
Senon Nordwestdeutschlands. Geol. Jahrb. 6~:
595-632.
HINKE, W. 1958. Das postembryonale relative Wach
stum des Gehirns and der wichtigsten Hirnteile yon
Drosophila melanogaster. Naturwiss. 24;:630-631.
HUXLEY, J. 1942. Evolution. The Modern Synthesis.
London.
1957. The three types of evolutionary process. Nature
180: 454-455.
J~RISON, H. J. 1955. Brain to body ratios and the evolution of intelligence. Science 121 : 447-449.
KLEIST, K. 1934. Gehirnpathologie. In O. von Schjerning, Handb. d. /*rztl. Erfahrungen im Weltkriege,
Bd. IV., Leipzig.
LAPIQUE,L. 1898. Sur la relation du poids de l'encephale
au poids du corps. C. r. Soc. Biol. X : s. 5.
1907. Les poids encephaliques en fonction du poids
corporal entre individus d'une m~me esp~ce. Bull.
Soc. Anthropol. Paris V, s. 8: 313-345.
LEINEMANN,K. 1904. l~ber die Zahl der Facetten in den
zusammengesetaten Augen der Coleopteren. Diss.
Miinster (Westf.), Hildesheim.
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
302
RENSCH
MOLLER, A. 1950. Die Struktur des Auges bei Urodelen
verschiedener K6rpergr6i~e. Zool. Jahrb. Abt. allg.
Zool. 62: 138-182.
MtiLLER, R. Vergleichende Untersuchungen fiber die
Gr6fienverh~ltnisse des Ohrlabyrinths einheimischer Siiugetiere. Gegenbaur's Jahrb. in press.
NEDER, R. Allometrisches Wachstum yon Hirnteilen bei
drei verschieden groi~en Schabenarten. Zool. Jahrb.
Abt. allg. Zool., in press.
NEUMANN, G. H. Die visuelle Lernf~higkeit primitiver
Si~ugetiere und VSgel. Z. f. Tierpsychol., in press.
PARTMANN,W. 1948. Untersuchungen fiber die komplexe
Auswirkung phylogenetischer K6rpergrSl~en~nderungen bei Dipteren. Zool. Hahrb. Abt. Anat. 69:
435-558.
QUIRING, D. P. 1938. A comparison of certain organ and
body weights in some African ungulates and the
African elephant. Growth 2: 335-346.
RATZENDORFER, C. 1952. Volumetric indices for the
parts of the insect brain. A comparative study in
cerebralization of insects. J. New York Entom. Soc.
60: 129-152.
REE'rZ, W. 1958. Unterschiedliehes visuelles LernvermSgen von Ratten und Mafisen. Z. f. Tierpsychol.
t4: 347-361.
R~NSCH, B. 1943. Die paliiontologischen Evolutionsregeln in zoologischer Betrachtung. Biologia Generalis
17: 1-55.
1954. Allgemeine Probleme der Abstammungslehre.
Die transspezifische Evolution. Stuttgart (Enke)
1 ed. 1947, 2 ed. 1954.
1949. Histologische Regeln bei phylogenetischen
KSrpergr56eniinderungen. (Symposium). La Ricerca Scientifica, Suppl. 19.
1954. Neuere Untersuchungen tiber transspezifische
Evolution. Verh. Deutsch. Zool. Ges (Freiburg
1952), 379-408.
1956. Increase of learning capability with increase of
brain-size. Amer. Naturalist, 90: 81-95.
1958. Die Abh~ngigkeit der Struktur und der Leistungen tierischer Gehirne yon ihrer Gr56e. Naturwiss.,
$5: 145-154.
1959a. Homo sapiens. Vom Tier zum Halbgott. GSttingen: Vandenhoeck u. Ruprecht.
1959b. Evolution Above the Species Level. New York
and London.
RENSCH, B. u. ALTEVOGT,R. 1955. Das Ausma6 visueller Lernfiihigkeit eines Indischen Elefanten. Z. f.
Tierpsychol. 12: 68-76.
SAXENA, A. 1959. Lernkapazit~t, Ged~chtnis, und
TranspositionsvermSgen bei Forellen. Diss. Mfinster
(Westf.)
SHARIFF, G. A. 1953. Cell counts in the primate cerebral
cortex. J. Compar. Neurol. 98: 381-400.
SIMPSON, G. G. 1949. The Meaning of Evolution. New
Haven.
1953. The Major Features of Evolution. New York.
SI-IOLL, D. 1947. Allometry of the vertebrate brain.
Nature: 159: 269-271.
SNELL, O. 1891. Die Abhiingigkeit des Hirngewichtes
yon dem KSrpergewicht und den geistigen Fi~higkeiten. Arch. Psychiatrie, 23: 436-446.
SPATZ, H. 1950. Menschwerdung und Gehirnentwicklung. Nachr. Giessener Hochschulges. 90: 32-55.
SPINA]~RANCANETTO,A. 1951. Dimensioni e numero dei
neuroni in relazione alia mole sistematica. Z. f.
Zellforseh. 36: 222-234.
SYaINO, A. 1957. Die Verbreitung von Spezialzellen in
der Gro6hirnrinde verschiedener S~ugetiergruppen.
Z. f. Zellforsch. $5: 399-434.
TOWER, D. B. 1954. Structural and functional organization of mammalian cerebral cortex: the correlation
of neuron density with brain size. Journ. Comp.
Neurol. 101: 19-52.
WAGNER,H. 1933. fJber den Farbensinn bei Eidechsen
Z. vergl. Physiol. 18: 379-436.
WATSON, D. M. S. 1949. The evidence afforded by fossil
vertebrates on the nature of evolution. In: Jepsen,
Mayr, and Simpson, Genetics, Paleontology, and
Evolution, 45-63. Princeton.
WIRZ, K. 1950. Studien fiber Cerebralisation zur quantitativen Bestimmung der Rangordnung bei S~ugetieren. Acta Anatomica 9: 134-196.
WOJTUSIAK,R. J. 1933. ~ber den Farbensinn der SehildkrSten. Z. vergl. Physiol. 18: 393-436.
1934. ~ber den Formensinn der SchildkrSten. Bull.
Ac. Polon. Sci. Lett. 349-373.
DISCUSSION
EMERSON: These investigations of the evolution of
brain function should be continued and extended,
but I think it m a y be too early to generalize from
the insects.
In the case of the termites, progressive evolution
of social behavior is predominantly associated
with a decrease in size, the most primitive forms
being large, and the majority of the derived forms
being small.
We have very little data on the functions of
the central nervous system of insects, and generalizations from brain size of vertebrates with highly
developed capacity for associational learning
across to organisms with a high degree of instinct
is questionable.
RENSCH: I t is a pity that we do not know enough
about the brain structure of termites. Hence m y
considerations could only be based on European
bees, wasps, and beetles. I n European species of
bees and wasps only the larger species show
complicated social instincts. I know well that
there exists also rather small social Hymenoptera
in the tropics. I t would be of great interest to
investigate the brain structure of these animals.
I t could be possible that the number of brain
cells is very large although the brain as a whole
is absolutely small. I n ants, too, the smallest
genus, Monorium, shows the simplest instincts.
On the other hand there are large species like in
Camponotus having comparatively simple instincts. I would not say that the complication of
instincts in insects always runs parallel with
absolute brain size. But I believe that species with
small brains and relatively few brain cells normally have less complicated instincts than related
and biologically comparable larger species, which
m a y have more complicated instincts or m a y also
have simpler instincts if their mode of life does
not require a complicated behavior. Of course
this can only be a working hypothesis.
CURTIS: I wish to congratulate Dr. Rensch and
his collaborators on a fine piece of work. I hope
that their efforts will continue in their present
direction.
M a y I ask for clarification of the following
point. The exclusive use of serial visual discrimination tasks in most of the studies suggests a
possible confronting of learning ability and visual
acuity. Larger animals presumably have more
perceptor elements in the retinal fovea, and m a y
have better resolution of visual patterns. If so, the
tasks are not of equal difficulty for large and
small animals. Auditory, tactile, and propriocep-
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
PROGRESS OF BRAINS AND SENSE ORGANS
rive discrimination suggest themselves. Have
you used other discrimination learning tests? You
mentioned the oddity problem. Have you systematically studied size difference by means of
Horten's delayed response found by Pribram
and by Harlow to be a very useful measure of
comparative brain function.
RENSCH: YOU are right in presuming, that larger
animals have more sense cells than closely related
smaller species. We could show this by countings
in the retina of newts and salamanders. However,
the difference is not very great and probably
does not affect the visual acuity. For example
in our experiments, the learning boxes and the
distance of the animal from the learned pattern
were more or less proportionate to body size and
the size of the patterns proportional to the diameter of the eye.
We have not yet made experiments with delayed responses but one of my collaborators
who performs transposition experiments with
large and small races of domestic fowl, will do it
later on.
SKERL:Working on a paper on human evolution I
303
found in Matiegka's book (1937) a quotation
which supports the graph showing the increasing
frontal part of the brain ill a modern Negro vs.
the Australopithecus. It is, according to Well, that
the surface area of the brain in front of the vertical
line touching the frontal edge of the temporal
lobe, was in Neanderthalers 21-25 per cent only,
whereas in Modern Man it is 28-31 per cent of
the whole brain surface area.
VAN VALEN: DO you know whether, or how much,
the relationship of brain size to learning ability in
related animals is due to the number of neurons,
as might be expected, or to their size?
RENSCH: Among mammals, especially in rodents,
the number of brain neurons seems to be very
similar in related smaller and larger species.
Especially the Dutch histologist Brummelkamp
made corresponding countings. However, larger
species have larger neurons with more dendritic
ramifications and this may allow more complicated patterns of excitations and more complicated associations. Among insects the larger
species have many more neurons than related
smaller species.
Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory
Press
Trends Towards Progress of Brains and Sense
Organs
Bernhard Rensch
Cold Spring Harb Symp Quant Biol 1959 24: 291-303
Access the most recent version at doi:10.1101/SQB.1959.024.01.027
References
This article cites 35 articles, 1 of which can be accessed free
at:
http://symposium.cshlp.org/content/24/291.refs.html
Email alerting
service
Receive free email alerts when new articles cite this article sign up in the box at the top right corner of the article or click
here
To subscribe to Cold Spring Harbor Symposia on Quantitative Biology go to:
http://symposium.cshlp.org/subscriptions
Copyright © 1959 Cold Spring Harbor Laboratory Press